The rising concerns about CO2 emissions have led to a growing realization that it is not possible to sustain the world's current development based on fossil fuels without a substitution of clean and renewable energy. Hydrogen, in addition to being an important chemical feedstock in global industry, is now firmly considered as one of the most likely future fuels. However, current hydrogen production primarily relies on the steam methane reforming process which is neither sustainable nor favored because the process requires high energy (heat) input and produces CO2 as a by-product. It is widely believed that room temperature electrochemical reduction of water to molecular hydrogen offers a significant promise for supplying CO2-free hydrogen, which can be used directly as a fuel or as reactant to convert CO2 and to upgrade petroleum and biomass feedstocks to value-added chemicals and fuels through hydrotreating processes. All these applications require large-scale, commercial processes for water electrolysis, which in turn require breakthrough discoveries in at least two areas: (1) the availability of electricity derived from renewable energy sources, such as solar and wind, and (2) the discovery of low-cost electrocatalysts to replace precious metals that are currently the state-of-the-art HER catalysts.
HER in an acidic environment generally requires lower overpotentials than those in a basic environment. However, a hydrogen production system in a basic environment is still more promising, because of the possibility to consider non-precious-metal based catalysts that cannot be used in acidic conditions, not only for HER at a cathode, but also for oxygen evolution reaction at an anode. Regardless of acidic or basic conditions, Pt, along with its alloys, is the benchmark electrocatalyst that requires very small overpotentials to drive the reaction, while the scarcity and high cost of Pt hinder its large-scale use for H2 production. As a result, enormous research efforts have been devoted to finding and engineering low-cost alternative catalysts. For example, tungsten and molybdenum carbides and sulfides, nickel phosphides, and electrodeposited Ni—Cu alloy have been identified as potential electrocatalysts for HER, but unfortunately most of these catalysts exhibit poor intrinsic activity and/or stability in strong bases.
Over the past decade, density functional theory (DFT) predictions, in conjunction with experimental efforts, have played a pivotal role in providing design principles of electrocatalysts. For hydrogen evolution, the activities (in terms of exchange current density) of different catalytic surfaces can be correlated with their hydrogen binding energy (HBE) via a volcano-type relationship, revealing that an optimal HBE would lead to the highest activity. Monometallic catalysts have been studied extensively using the DFT method. However, monometallic non-precious metals show HBE values significantly different from that of Pt.